Heat engines use energy provided in the form of heat to do work. There are several thermodynamic cycles that may be employed by heat engines such as, for example, the Carnot cycle, the Stirling cycle, and various internal combustion engine cycles like the Otto cycle and the diesel cycle. All of these thermodynamic heat engines use a gas as the working fluid. For example, the Stirling cycle is often used in relatively small and modest sized cryocoolers, where the working fluid is usually helium.
Stirling engines may include internal pistons that are used for displacing and compressing the working fluid and to generate output power. Specifically, the pistons receive work during their up-stroke or compression, and generate work during their down-stroke or expansion, followed by a transfer of heat at a given temperature by the working fluid to a surrounding heat sink. The pistons of the Stirling engine may be actuated by suspending the pistons using flexure bearings, and then creating the driving motion using electromagnets. However, the electromagnets create a driving force that is predominantly sinusoidal in time. The sinusoidal driving force is caused by the inductance of the electromagnet's coils as well as because the drive voltage and the switching speeds are both kept relatively low.
Rotating machinery also includes a stator and a rotor. In at least some applications, the rotor may need to be cooled. In order to cool a rotor, cooled gas from a reservoir or refrigerator may be introduced in a space between the rotor and a stationary component such as the stator. However, the rotor is cooled unevenly since the outermost surface of the rotor experiences a majority of the cooled gas. Therefore, in order to cool the interior of the rotor, special rotating joints for passage of the cooling gas may be required. Furthermore, if a refrigerator is used to store the cooled gas, then electrical power connections are required.
Electrical power is produced on a rotor by contacting the rotor with brushes that carry electrical current. Electrical current generated on a component that is not the rotor may flow through the brushes and along electrical conductors upon the rotor, where the electrical current is then utilized. However, the passage of the electric current through the electrical conductors produces heat, which in turn creates Joule heating losses. Furthermore, the brushes may contact a shaft of the engine, which results in wear and maintenance issues. The wear created by the brushes increases with rotational speed. In another approach to provide power to the rotor, a battery may be used instead of brushes. However, batteries only store a limited amount of energy and eventually need to be recharged or replaced. Finally, in yet another approach electrical power for the rotor is produced by induction transfer from coils located on the stator to coils located on the rotor. However, Joule heating losses occur in the coils on both the rotor and the stator.